An enormous amount of effort has been expended in attempts to elucidate the underlying mechanisms controlling flower development in various dicotyledonous plant species (reviewed in Coen, Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:241–279, 1991; and Gasser, Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:621–649, 1991), leading to the isolation of a family of genes which encode certain regulatory proteins. The most studied plant regulatory proteins identified to date include AGAMOUS (AG) (Yanofsky et al., Nature 346:35–39, 1990), APETELA I (API) (Mandel et al., Nature 360:273–277, 1992), and APETALA 3 (AP3) (Jack et al., Cell 68:683–697, 1992) in Arabidopsis thaliana, and DEFICIENS A (DEF A) (Sommer et al., EMBO J. 11:251–263, 1990), GLOBOSA (GLO) (Trobner et al., EMBO J. 11:4693–4704, 1992), SQUAMOSA (SQUA) (Huijser et al., EMBO J. 11:1239–1249, 1992), and PLENA (PLE) (Bradley et al., Cell 72:85–95, 1993) in Antirrhinum majus. 
Sequence analysis of these plant regulatory genes has revealed that their gene products contain a conserved MADS-box region (Bradley et al., Cell 72:85–95, 1993; Huijser et al., EMBO J. 11:1239–1249, 1992; Jack et al., Cell 68:683–697, 1992; Mandel et al., Nature 360:273–277, 1992; Sommer et al., EMBO J. 11:251–263, 1990; Trobner et al., EMBO J. 11:4693–4704, 1992; Yanofsky et al., Nature 346:35–39, 1990). Transgenic approaches have been employed to study the functional roles of MADS-box genes (Kempin et al., Plant Physiol. 103:1041–1046, 1993; Mandel et al., Cell 71:133–143, 1992). MADS-box genes have been found to play an important role in specifying floral meristems and floral organ identity in plants such as Arabidopsis (Yanofsky et al., The Plant Cell, 7:721–733, 1995). Furthermore, using conserved MADS-box regions as probes, MADS-box genes have been isolated from other species including tomato (Mandel et al., Cell 71:133–143, 1992), tobacco (Kempin et al., Plant Physiol. 103:1041–1046, 1993), petunia (Angenent et al., Plant Cell 4:983–993, 1992), Brassica napus (Mandel et al., Cell 71:133–143, 1992), and maize (Schmidt et al., Plant Cell 5:729–737, 1993).
However, very few MADS-box genes have been cloned from tree species. MADS-box genes were recently found in spruce trees to encode transcription factors, which are key components in the developmental control systems of conifers, such as the identity of the floral organs (Tandre, et al., The Plant Journal 15(5), 615–623, 1998). Also, recently another MADS-box gene, a DEFICIENS homolog and its promoter were isolated from the dioecious tree, black cottonwood and were found to regulate expression in female and male floral meristems of the two-whorled, unisexual flowers (Strauss et al., Plant Physiology 124:627–639, 2000).
In addition, genetic engineering is slowly showing potential for the improvement of qualitative and quantitative traits in plants and trees. However, the full potential of transgenic plants can not be realized until methods can be developed to restrict or eliminate long distance migration of seeds and pollen from transgenic plants. Accordingly, to facilitate the production of genetically engineered trees, it is desirable that the trees be completely reproductively sterile. The ability to produce sterile transgenic trees is very desirable because there is a high potential for escape of transgenes in trees into wild populations due to their long distance movement of seeds, pollen, and their ubiquitous wild relatives. Engineering total male and female sterility for gene containment and other desired traits or partial sterility for selective breeding as discussed herein below is an important characteristic for transgenic plants to possess. It would therefore be highly desirable to have means to affect complete or partial reproductive sterility. The present invention satisfies this need and provides related advantages as well.
Also, timing of the transition from vegetative growth to flowering is one of the most important steps in plant development. This determines quality and quantity of most crop species since the transition determines the balance between vegetative and reproductive growth. It would therefore be highly desirable to have means to affect the timing of this transition so that flowering time can be controlled resulting in earlier flowering and conversion of axillary meristems to floral meristems.
Furthermore, traditionally, plant breeding involves generating hybrids of existing plants, which are examined for improved yield or quality. The improvement of existing plant crops through plant breeding is central to increasing the amount of food grown in the world since the amount of land suitable for agriculture is limited. For example, the development of new strains of crops, fruit and nuts through plant breeding has increased the yield of these crops grown in underdeveloped countries. Unfortunately, plant breeding is inherently a slow process since plants must be reproductively mature before selective breeding can proceed. For some plant species, the length of time needed to mature to flowering is so long that selective breeding, which requires several rounds of backcrossing progeny plants with their parents, is impractical. For example, some trees do not flower for several years after planting. It would therefore be highly desirable to have means to affect breeding of such plant species for a variety of different economically valuable or aesthetically pleasing traits.